1. Field of the Invention
This invention relates generally to turbine blade design and, more particularly, to an improved free standing turbine blade having improved mechanical reliability.
2. Description of the Related Art
A steam turbine can include a combination of low pressure, intermediate pressure, and/or high pressure steam turbine elements which are coupled together to provide a single power output. Each steam turbine includes a rotor having a plurality of rotating blades mounted thereon in grooves. Usually, the blades of a given row are identical to each other. The rotating blades of a row extend radially outwardly from an outer surface of the rotor, with the rows being spaced apart. The rotating blades of one row differ in shape from those of the other rows; most noticeably the rotating blades of each row, or stage, vary in length depending on position along the rotor.
Each rotating blade, regardless of row, has a foil portion extending radially outwardly from the rotor and a base portion for mounting the blade to the rotor. The base portion includes a root which is fitted into a mounting groove provided for each blade of a row, and a platform integrally formed at the proximal end of the foil portion. The foil portion has a tip at the distal end and may have a twist profile from the proximal end to the distal end, or may be parallel-sided. Sometimes, shrouds are provided at the tips as separately added or integrally formed components.
A stationary cylinder is coaxially supported around the rotor and has a plurality of stationary blades mounted on an inner surface thereof. The stationary blades are arranged in rows which, when the cylinder is assembled with rotor, alternate with rows of rotating blades. The stationary blades of one row are shaped differently from those of the other rows, although all stationary blades have a foil portion. Some stationary blades have a base portion which includes a root and a platform. Other have the foil portion welded directly into blade rings with no root or platform.
The root of each stationary blade may be provided with a side notch, which when the root is fitted into the groove, aligns with an annular recess. The side notch and the annular recess together define a space which is common to both cylinder and the root. When the space is filled with caulking material, the cylinder and root become interconnected.
Rotor blade grooves provided in the rotor for mounting the rotor blades are usually geometrically more complex than the mounting grooves provided for stationary blades. Moreover, the roots of the rotating blades and the rotor are subjected to substantially greater stresses than corresponding roots of stationary blades.
Some turbines have turbine rotor blades mounted in what are referred to as "side-entry" grooves provided in the rotor. When mounted, the rotor blades extend radially outwardly from the rotor in rows which are disposed circumferentially around the rotor. Instead of having a single annular groove for mounting the plurality of rotor blades which constitute a row, a side-entry groove arrangement includes, for a given row, a series of equidistantly spaced apart side-entry grooves, each side-entry groove of the series being provided for each rotor blade of the row. Although the side-entry grooves are usually equidistantly spaced, sometime spacing is varied to facilitate assembly of a closing blade.
A typical side-entry groove starts at the outer surface of the rotor as an opening which tapers inwardly towards a bottom of the groove. A series of undulations are provided between the opening and the bottom of the groove symmetrically on opposite sidewalls of the groove. A typical root of a corresponding turbine blade has a shape which substantially conforms to that of the groove. The undulations provide a series of interlocking steps. The undulating sidewalls also make it possible to insert the root into the groove radially relative to the rotor. The resulting shape of the rotor grooves and blade root is sometimes referred to as an inverted fir tree or steeple.
In a side-entry groove, the root is pushed into the groove substantially parallel to the turbine rotor axis, and therefore, an interlocking can be achieved. Tolerances for both root and groove are very precise. A root contour tolerance envelope for contact surfaces typically varies along the contour root from one to five ten thousandths of an inch. A groove contour tolerance envelope for contact surfaces typically varies along the profile of the groove from about six to eight ten thousandths of an inch. Basically, a precise fitting between the root and the groove is required such that the maximum clearance between the root and the groove is extremely small.
There is a general reluctance to change rotor blade root and groove configurations once a particular design has been developed. This is because it may have taken months or even years of meticulous calculation to arrive at a particular design. Sometimes, slight variations in rotor blade root and groove profiles lead to unacceptable decreases in the function or performance of the blades or the rotor. Given that the tolerances between the root and the groove are critical, changes in the profile of either or both goes against conventional wisdom.
Ordinarily, the root of a side-entry rotor blade fits into the groove which has a shape nearly identical to that of the root. This is done in order to minimize losses associated with leakage of the motive fluid. An exception to this practice sometimes occurs in high-temperature applications, where clearances are introduced between the bottom of the root and the bottom of the groove to provide a passage through which a cooling medium can pass.
Fir-tree blade roots and their corresponding mounting grooves are widest at their locations nearest to the foil and the rotor body, respectively, and narrowest at the opposite ends. This is done in order to most efficiently exploit the material which is available to transmit loads from the blade to the rotor, and to provide for generous fillet radii which serve to minimize stress concentration effects.
Power generation units will over time require replacement of the blades of the turbine. Frequently, customers request that the power generation units be upgraded in terms of performance by retrofitting blades having higher performance characteristics. Present markets emphasize replacement blading on operating units to extend life, to obtain the benefits of improved thermoperformance, and to improve reliability. In addition, upgraded versions of current turbine designs with improved reliability and performance are required.
It has been observed that currently used free standing blades develop cracks in the root neck during cyclic duty operation. These root cracks are caused by repeated start-up and shut down cycle. One of the principle reasons for the development of root or steeple cracking is trailing edge "overhang" with respect to the root neck. Trailing edge overhang is illustrated in FIG. 1 as the distance A between the trailing edge 38 of a blade at the base thereof and an outline 12 of an area of the root defined by the uppermost root neck. This area is also shown in FIG. 2, which is a combination view which includes sectional and side elevational aspects. In FIG. 2, the root portion 46 and the uppermost root neck 48a are illustrated. FIG. 2 is a blade of slightly different configuration than FIG. 1, and illustrates a slightly more pronounced trailing edge overhang.
FIG. 3 is a stacked composite showing blade sections A--A, E--E, J--J and M--M, as well as the base section, section Q--Q which shows the relationship of the platform portion 44 to that particular section.
The various sections illustrate the contour of the blade as it progresses from the base section (Q--Q) to the tip section (A--A). Each section illustrates the basic components of the blade, which are the leading edge 36, the trailing edge 38, the convex, suction-side surface 40 and the concave, pressure-side surface 42. The root portion 46 is shown to the left-hand side of section Q--Q, for illustrative purposes as having a root center line bisecting the root portion 46 about a vertical plane of symmetry. Although the trailing edge 38 is shown in FIG. 3 to be not far from the edge of the platform portion 44, its position relative to the uppermost root neck 30 is considered critical to the present invention.